Coupled Bunch Mode Instabilities Research Articles

On Landau Damping by Bunch Filling Pattern in the Pohang Light Source Storage Ring

The stable beam current of the Pohang Light Source (PLS) is normally limited to about 150 mA despite the design value of 400 mA at 2.0 GeV due to the coupled bunch mode instabilities (CBMIs) which are generated by the interaction between higher order modes of RF cavities and circulating beams. In order to improve this situation, a longitudinal feedback system (LFS) using parallel digital signal processors was installed during the summer maintenance period in 1999. Besides the cure of the CBMIs, this programmable LFS is useful for various beam diagnostics such as the synchrotron oscillation phases and their spreads, and the beam pseudo-spectra that are the beam spectra without beam revolution harmonics. With this bunch-to-bunch diagnostic information, it is found that properly arranged bunch filling patterns can increase the stored beam current significantly without an active longitudinal feedback due to Landau damping by bunch-to-bunch synchrotron frequency spreads.

Longitudinal feedback system kicker for the PLS storage ring

The higher order modes (HOMs) of RF cavities at the Pohang Light Source (PLS) storage ring cause longitudinal coupled bunch mode instabilities. A longitudinal feedback system (LFS) is introduced to cure these instabilities. As a key component of the LFS, there is a single-ridged waveguide-overloaded cavity for a longitudinal bunch-by-bunch LFS kicker. Since the RF frequency is 500 MHz, the bandwidth of this kicker should be wider than 250 MHz to damp any coupled bunch mode. Also, a higher shunt impedance of the kicker is selected to use a lower power amplifier. One aluminum kicker is fabricated, and the bandwidth, HOMs, and the shunt impedance of the kicker are measured with a network analyzer. This kicker has several different features from the LFS kicker for DA/spl Phi/NE. First of all, there are four input/output ports to obtain a wider bandwidth. Second, there is a nose cone to obtain a higher shunt impedance. Third, the symmetric frequency response of the shunt impedance around the central frequency is provided. Finally, low beam coupling power (4 W per port at 400 mA) is obtained. According to the simulation result by the high frequency structure simulator (HFSS) code, we obtained the high shunt impedance of 621.3 /spl Omega/ (transit time factor considered value) and the wide bandwidth of 255 MHz. These are compared with the measured result.

Longitudinal Instabilities of Bunched Beams in the ISR

Microwave instabilities occur in bunched beams in the ISR leading to a dilution of the phase space density and limiting the longitudinal density of the stacked beams. According to D. Boussard this instability can be described as a coasting beam instability inside bunches. Experimental investigations of this microwave instability support this theory and give a high frequency impedance |Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> |/n ≈ 14 ohms. Injecting large currents in bunches of large area increases the threshold of this instability. The larger currents can produce coupled bunch mode instabilities which can be cured by a higher harmonic cavity.